1. Field of the Invention
The present invention relates to a device for correcting the phase induced by the class C operation of a solid state amplifier. It is applied in the field of the manufacture of pulse radar transmitters.
2. Background Discussion
In modern radars, the transmitted electromagnetic waves are generated by a device based on semiconductor components using microwave frequency transistors. The characteristics of the radar are directly related to the transfer function. Now, the latter depends on the operating conditions of the system.
In most radar applications, the Doppler effect affects the signals returned by moving targets, allowing the elimination of fixed echoes. Thus, in the case of a pulse compression radar, a frequency shift due to the Doppler effect is added to the phase or frequency encoding.
Now, the quality of the pulse compression radar essentially depends upon the ability to provide a receiving system whose transfer function is strictly the reverse of that of the transmission system. When working with solid state power stages, the amplification stages work in Class C. The peak powers are lower than in the case technologies based on valves (klystrons, Standing Wave Tubes, etc.) but the medium power developed can be high in order to allow a suitable form factor.
The desired powers are obtained by grouping power stages, controlled by low level transistors, in cascade. The final power is obtained by adders (Wilkinson circuits, 3 dB rings, . . . ). These circuits always allow a sufficient power to be maintained even in the case of the failure of several stages which are automatically replaced.
However, power semiconductors are subject to two types of negative phenomena when used in pulse radars. Firstly, the instantaneous power varies during the pulse. Then, the phase varies.
During the progress of the invention, it was discovered that these phenomena were due to considerable variations in the junction temperature of the power transistors in the final stages. In pulse class C, the junctions change from zero dissipation before the rise of the pulse to maximum dissipation when the pulse power is at its maximum value. Now, the efficiency of transistors in this range being of the order of forty per cent, the heat dissipation is high during the duration of the pulse. The junction/casing thermal resistance of each transistor, however low it may be, results in the temperature varying during the pulse instead of remaining constant according to perfect radar theory.
As, in addition, the radars can work in frequency excursion mode or in frequency diversity mode or in a random wobbulated frequency mode to avoid countermeasures, the temperature variations become inhibitory.
In fact, they are responsible for variations in the phase and amplitude of the transmitted signal. In Doppler processing, the problem is further increased by the fact that the radar creates a band of parasitic Doppler frequencies.
The obvious solution which consists in cooling the junctions of the transistors during the relaxation between two pulses cannot be applied here since the widths of the pulses and the relaxation durations can vary considerably with no direct relationship between them.